1. Field of Invention
This invention relates to a method and apparatus for measuring body fat for measuring the amount of fatty tissue in vivo.
2. Description of the Prior Art
It has been customary to determine whether or not a human being is fat from the relationship between body weight and height.
However, it is not always unhealthy that weight is large in comparison with height, and it is important to measure fatty tissue in vivo. It is practical to measure body fat in vivo by measuring body weight. Recently, various body fat measuring instruments have been sold in the market.
An underwater body weighing method is known for estimating body fat by calculating body density from the measured weight value under water as a method for accurately measuring body fat in vivo.
This underwater body weighing method needs a large facility, and necessitates skillful measuring techniques and an effort by the person being measured considering the large influence of the amount of residual air in the lungs to be measured.
In order to eliminate the disadvantages of the above-described underwater body weighing method, there has been proposed a method for estimating body density by measuring body impedance between extremities of the body and estimating the body density from the measured impedance value, height and body weight.
The ratio of body water in fat-free tissue of body composition tissue is constant and the specific resistance of the fat-free tissue is constant. The estimation formula: body density=1.1554 -0.0841*weight*impedance/(height).sup.2 of Segal et al., and body formula: density=1.1303--0.726*weight*impedance/(height).sup.2 of Naka et al., by obtaining the inference formula as represented by body density=A-k*weight*impedance/(height).sup.2 and obtaining correlation between the constant A and the value obtained actually by an underwater body weighing method for determining the constants A and k, are disclosed.
However, it is known that the impedance value varies due to various factors such as variation in body water ratio in fat-free tissue due to variation in body weight, amount of blood plasma due to variation in motion or attitude and movement of interstitial liquid. In order to accurately measure the impedance value, it is necessary to consider influences of ingestion of food and water, motion, attitude and variation in body weight.
Thus, the impedance value of a body to be measured vary according to the date, time and differences of body condition before measurement. These various are too large to use the above-described inference formula accurately and amount of body fat calculated by the formula is likely to be inaccurate.
Therefore, in order to measure accurately, it is necessary to maintain the body state of the patient to be measured constant by selecting a day for taking the measurements when ingestion of food and water, and activity, are limited and variation in body weight is small, and to measure the impedance value after the patient has been lying on a bed for a predetermined period of time. Thus, the measuring conditions are known.
On the other hand, there has also been proposed a method for calculating body fat from numeric values with respect to a body such as an impedance between extremities of the body, height, weight and gender by measuring the impedance using a 4-terminal Kelvin bridge living body impedance meter as a method for accurately and simply measuring the body fat in vivo as in U.S. Pat. Nos. 4,008,712 and 4,895,163 (Japanese Patent Laid-open No. Hei 2-60626).
Since the degrees of bending and the angle of the limbs influence the impedance values using these methods, the measuring conditions when the patient is lying on a bed are maintained constant to eliminate errors.
Further, if the spacing between a current supply terminal and a voltage detecting terminal of electrodes for detecting the impedance is not sufficient, the measured values are affected, and hence the spacing is maintained by mounting the current supply terminals on the backs of the hand and foot and mounting the voltage detecting terminals on the tendons of the wrist and the ankle.
Accordingly, restrictions in the measuring position are significant (needs a bed, etc.), and it is difficult for a person to use the apparatus to take measurements of himself without assistance.
Moreover, in a method for measuring living body impedance, it has also been necessary to measure body fat as described above, using a method having steps of outputting a sine wave of 50 kHz from an oscillator as a constant-voltage source, converting it to a constant current of 800 microamperes by a voltage/current converter, supplying the current through a pair of electrodes mounted on a body, outputting the voltage value of a voltage drop from a pair of electrodes mounted inside the above-described electrodes by a differential amplifier, waveform-shaping, rectifying it, DC-converting it, then, A/D-converting it as digital data, and applying it to a calculator, determining the living body impedance Z by Z=V/I from the living body current I and the terminal voltage V, and measuring the voltage V when the current I is constant, thereby determining the living body impedance Z.
However, the constant-current source is affected and its output is varied due to the influence of the external environmental temperature, etc., by the degree of variation in the electrode impedance, and it is difficult to obtain an accurate value of the living body current I as an accurate constant current, thereby causing error in measuring the living body impedance.
On the other hand, the living body impedance to be measured by a body fat meter needs to be accurate in a wide range of 0 to about 1 kilo-ohm. It is extremely difficult to obtain an ideal proportional relation in this range, thereby necessitating a great deal of work for making corrections to the measured value.
Accordingly, an object of this invention is to provide a method and apparatus for measuring body fat which has a small restriction in measuring conditions and which can be simply measured without influence by the measuring conditions.
Another object of this invention is to provide a practical body fat measuring apparatus which can be readily used by the person being measured without restriction.
Still another object of this invention is to provide a living body impedance measuring method which is particularly adapted for measuring body fat and can accurately measure the absolute value of the living body impedance in a wide measuring range without influence of the environment.